This invention relates to X-ray imaging systems, and more particularly the present invention relates to composition, density and geometry imaging of an object by measuring and analyzing incident and scattered radiation passing through that object.
The following publications relate to the subject of x-ray imaging systems and methods. Various teachings from these publications are cited herein to facilitate the description of the present invention.
Glasstone S. and Sesonske, A., Nuclear Reactor Engineering, Chapter 2, Chapman and Hall, New York, 1994.
Zieglier, C. A., Bird, L. L. and Chel{dot over (e)}k, xe2x80x9cX-Ray Raleigh Scattering Method for Analysis of Heavy Atoms in Low Z Mediaxe2x80x9d, Analytical Chemistry, Vol. 13, pp. 1794-1798, 1956.
Bray, D. E. and Stanley, R. K., Nondestructive Evaluation, Chapter 20, McGraw-Hill, New York, 1989.
Battista, J. J. and Bronskill, M. J., xe2x80x9cCompton scatter imaging of transverse sections: an overall appraisal and evaluation for radiotherapy planningxe2x80x9d, Physics in Medicine and Biology, Vol. 26, pp. 81-99, 1981.
Lale, P. G., xe2x80x9cThe examination of internal tissues, using gamma-ray scatter with a possible extension to megavoltage radiographyxe2x80x9d, Physics in Medicine and Biology., Vol. 4, pp. 159-167, 1959.
Hussein, E. M. A., xe2x80x9cCompton Scatter Imaging Systemsxe2x80x9d, in Bioinstrumentation: Research, Development and Applications, Butterworths Publ., Stoneham, M A, D. L. Wise, Ed., Chapter 35, pp. 1053-1086, 1990.
Harding, G. and Kosanetzky, J, xe2x80x9cScattered X-ray Beam Nondestructive Testingxe2x80x9d, Nuclear Instruments and Methods, Vol. A280, pp. 517-528,1989.
Prettyman, T. H., Gardner, R. P., Russ, J. C. and Verghese, K., xe2x80x9cA Combined Transmission and Scattering Tomographic Approach to Composition and Density Imagingxe2x80x9d, Applied Radiation and Isotopes, Vol. 44, pp. 1327-1341, 1993.
Arendtsz, N., V. and Hussein E. M. A., xe2x80x9cEnergy-spectral Scatter Imaging. Part I: Theory and Mathematicsxe2x80x9d, IEEE Transactions on Nuclear Science, Vol. 42, pp. 2155-2165, 1995.
Arendtsz, N., V. and Hussein E. M. A., xe2x80x9cEnergy-spectral Scatter Imaging. Part II: Experimentsxe2x80x9d, IEEE Transactions on Nuclear Science, Vol. 42, pp. 2166-2172, 1995.
MCNP 4C, Monte Carlo N-Particle Transport Code System, RSICC Code Package CCC-700, Oak Ridge National Laboratory.
Conventional x-ray radiographic systems, commonly used in airports to detect weapons, sharp objects and the likes, are not suited for the detection of plastic explosives. This is due to the fact that such systems typically utilize low-energy photons, where the photoelectric effect (photon absorption) dominates. The probability of photoelectric absorption per atom can be roughly expressed as follows as taught by Glasstone et al.:
xcfx84≅constant Zn/E3xe2x80x83xe2x80x83(1) 
where Z is the atomic number of the medium, E is the photon energy, and the exponent n varies between 3 for low-energy photons to 5 for high-energy rays. Therefore, the low Z-number of nitrogen-based explosives makes it difficult to distinguish them from other common materials, with the photoelectric effect on which conventional radiography relies. Alternative techniques were therefore developed.
If the Compton scattering modality of photons is allowed to come into play, then additional information can be brought in to assist in detecting explosives. The probability of Compton scattering per atom, "sgr" depends on the number of electrons available as scattering targets and therefore increases linearly with Z, and can be expressed as follows, as taught by Glasstone et al.
"sgr"=constant Z/Exe2x80x83xe2x80x83(2) 
Therefore, Compton scattering provides density-related information. The electron density is directly proportional to Z, and the mass density is proportional to the electron density (given that the ratio of the atomic-number to the mass-number is equal to about one-half for most elements, except hydrogen) as taught by Zieglier et al.
Compton scattering can provide such mass-density information, which if used in conjunction with the Z-number information given by the photoelectric effect can help in identifying nitrogen-based explosives; that are characterized by having higher mass density than most common organic materials. A dual-energy (high and low) radiographic system can be used for this purpose; with the higher energy providing electron-density information and the lower energy strongly reflecting the effect of the Z-number, according to equations (1) and (2). This is the concept of the E-scan system. Alternatively, scattering can be monitored, typically back scattering, to obtain density information.
Another useful photon-interaction modality is the coherent Rayleigh scattering process, where photons are deflected by a small angle without losing energy. The probability of this reaction is however small and is proportional to Z3 making the reaction more sensitive to metals, as taught by Zieglier et al.
X-ray fluorescence depends on the production of x-rays characteristic of the target atom. However the technique is best suited for high Z atoms, and even then the measured flux is low, as taught by Zieglier et al.
The remaining photon interaction of significance is pair production, where a high energy photon disintegrates into an electron-positron pair in the presence of the electromagnetic field of the atom. Once again, this is an interaction that dominates at high Z number and high photon energy, as taught by Glasstone et al. This leaves the photoelectric effect and Compton scattering as the most suitable photon-interaction modalities for use in imaging.
Radiography techniques are disadvantaged by the fact that they provide integrated information, along the chord of radiation transmission, thus mixing the attributes of overlying objects. This can lead to masking and smearing out of information. Computed tomography (CT) solves this problem by unfolding the radiation-attenuation measurements into pixel-specific information at individual slice of the object. While solving the masking problem of radiography, the fact still remains that CT determines the attenuation coefficient of the material present in the pixel. Therefore, at the commonly used X-ray operating range of 80-200 kV, keeping in mind that the average X-ray energy in keV is equal to about one-third the peak energy which corresponds to the operating voltage in kV as taught by Bray et al., the photoelectric effect dominates in CT, as can be seen by comparing the equations (1) and (2). Therefore, in essence one obtains physical information that is identical in nature to that obtained by basic radiography, although de-convoluted into individual pixels. This comes at considerable cost due to the involvement of a complex mechanical scanning mechanism and a sophisticated numerical image reconstruction process.
The question now is whether a material can be uniquely identified from the value of its attenuation coefficient, or CT number. This question has bewildered medical physicists who plan for radiotherapy (at high photon-energy where Compton-scattering is dominant) from CT images (produced by low-energy X-rays where the photoelectric effect prevails). With the known nature of the body, some empirical formulations are devised, relating CT numbers to the electron density of tissue, muscles and bones, as taught by Battista et al. Given however the wide variety of materials that may be present in a passenger luggage, CT numbers may not necessarily be uniquely related to density, thus resulting in ambiguous and perhaps false indications. Like the case with conventional radiography, more information is needed to uniquely identify an explosive material from CT images. Such information can come from a suspicious object geometry, or other non-technical supplementary information. Alternatively, one can expect CT to progress in the same fashion as conventional radiography to provide additional physical information.
Progress of CT as an explosive detection system (EDS) requires that it provides both density and Z-number information. This can be achieved, similar to E-scan, by using a dual-energy CT system; but this duplicates an already mechanically and numerically intensive process. Alternatively, one can rely on combination of scattering and transmission measurements to provide electron-density and Z-number information. Scattering can also enable the development of a simplified and less expensive (non-rotating) imaging system; which is more suited for imaging carry-on luggage and/or for use in small and remote airports where the cost of a CT system can be prohibitive. Rotation-scanning is also more difficult to perform on bulky objects, such as cargo containers; and hence there is a need for non-rotating imaging system.
A brief review of Compton-scatter imaging (CSI) is given hereinbelow to further facilitate the description of the present invention.
Scatter imaging resembles the natural imaging process in which the naked-eye constructs an image from light reflected off the surface of an object. Unlike light, radiation penetrates deep into the object enabling volume imaging. Imaging underneath a surface is complicated however by the attenuation of radiation prior to and following scattering. This attenuation effect complicates the imaging process and has hindered the progress of scatter imaging for many years. The history of imaging with scattered photons can be traced back to the work of Lale in 1959, who employed a high-energy source, and consequently ignored altogether the attenuation effect. Many other workers attempted to overcome this problem in a variety of ways, for example Hussein. Perhaps the most significant developments in CSI are those of Battista et al., Prettyman et al., Harding et al., and Hussein and Arendtsz, which are briefly described below.
Battista et al. employed a gamma-ray source and a rectilinear scanning process, which enabled the determination of each pixel""s density as the scanning process progressed. The density of a preceding pixel was used to calculate the attenuation coefficients of the subsequent pixel. The ComScan(trademark) system of Harding et al. utilizes a collimated X-ray source and a detector array equipped with pinhole-type collimator to measure back scattered radiation. No correction for radiation attenuation appears to be incorporated in ComScan(trademark), making the system suitable only for xe2x80x98imaging of superficial regions of massive objectsxe2x80x99. Prettyman et al. used gamma rays and employed a combination of tomographic transmission scanning and projected Compton scatter imaging to obtain composition (Z-number) and density images. This system, however, relied on a scanning/rotation process that involved a large number of projections. In the recent work of Arendtsz and Hussein, the scanning process was avoided altogether by measuring the energy of the scattered photons and relating it to the angle of scattering using the unique energy-to-angle relationship of single-collision Compton scattering. A gamma-ray source was used to provide a mono-energetic source, thus facilitating the process of relating the energy to the angle of scattering. An iterative image reconstruction process was employed by Arendtsz and Hussein to overcome the nonlinear problem of accounting for pre- and post-scattering attenuation.
The present invention consists of a non-rotating photon (X-ray) system and methods for three-dimensional, three-parameter imaging of objects, for the purpose of identifying non-intrusively their material content. This system and methods are useful, for instance, in detecting explosives, narcotics, or other contraband materials, in passenger luggage or shopped parcels. The system and methods provide simultaneously three independent physical properties that enables the classification of materials by density and overall composition, in addition to the shape information provided by a 3-D imaging process. The system and methods employ a collimated beam of photons emitted from an x-ray machine operating in the 300 to 400 kV range, and monitor radiation scattered to the sides of the object, along with transmitted radiation. A rectilinear scanning process moves the object in front of the radiation beam in small steps, until the entire object is covered with radiation, by penetration through only one of its surface (that facing the source).
This scanning process simplifies the imaging process and reduces its cost relative to conventional systems, by enabling the source and detectors to be fixed in place. The scanning process facilitates the use of a single transmission detector, and one-dimensional (line) arrays of scattering detectors. It also renders the imaging process into a simple point-by-point imaging process, wherein the measurements are readily mathematically formulated and numerically processed to reconstruct simultaneously three images: a) the radiation attenuation coefficient at the source energy, b) the attenuation coefficient at the scattering energy, and c) the electron density at each voxel. The difference in photon energy between the incident and the scattered photons makes the attenuation coefficient for the latter more sensitive to variations in the atomic number of the material than the attenuation coefficient at the incident energy. Therefore, this invention is unique in its tri-property imaging process; a feature not provided by any other imaging systems, and should provide higher confidence in detecting concealed objects.
One feature of the present invention is that it advances the progression of X-ray explosive detection systems (EDS) by developing a Compton-scatter/transmission system which provides both density and atomic-number images of scanned objects. The system and methods according to the present invention are particularly appropriate for luggage imaging whereby density, attenuation-coefficient and atomic-number images, are obtained in a manner that requires exposing the luggage to radiation from only one side thereof, thus eliminating the need for a rotating scanning mechanism. This is done by combining the best features of various Compton-scatter-imaging approaches into a system suitable for luggage imaging while employing the rectilinear scanning process of Battista et al.; a source-detector arrangement similar to that of Harding et al. ; single-projection transmission measurements to provide density and Z-number information in a manner similar to that of Prettyman et al., and simplified forms of the mathematical formulations and image reconstruction algorithms as taught by Arendtsz and Hussein.
By eliminating the source/detector, or object, rotation process of transmission CT systems, a simplified system has been developed, with decreased mathematical and numerical complexity. The system and methods according to the present invention should enable more wide-spread installation of EDS in airports. The simplification process comes at the added advantage of supplying not only attenuation coefficients (CT numbers), but also providing atomic-number information (through the ratio of the attenuation coefficient of the photoelectric effect to that of Compton scattering), as well as atomic number images (through Compton scattering). This is in addition to the spatial imaging information that can enable the identification of the geometry of a concealed object. With a sufficiently small voxel size, the method may also enable the detection of sheets of explosives. The system and methods according to the present invention advance CT technology in the same way radiographic technology progressed, through the E-scan and back scattering concepts, to meet the demands of explosive detection.
Broadly, in accordance with one aspect of the present invention, there is provided a system for inspecting an object, comprising; a structure having a first, second and third orthogonal axes, and a source of collimated x-ray pencil beam mounted thereto along the first axis. The system also comprises an incident radiation detector mounted to the structure perpendicularly to the first axis; a first linear array of scattered radiation detectors mounted to the structure perpendicularly to the second axis, and a second linear array of scattered radiation detectors mounted to the structure perpendicularly to the third axis. The source of collimated x-ray pencil beam, the incident radiation detector and the first and second linear arrays of scattered radiation detectors being spaced apart and defining therebetween an inspection zone. The system according to the present invention further has means for moving an object to be inspected, relative to the source of collimated x-ray pencil beam, mounted to the structure in the inspection zone.
This system is particularly advantageous for being relatively simple and wherein the radiation measurements available therefrom when inspecting a voxel in an object are indicative of incident radiation attenuation, scattered radiation attenuation and electron density of that voxel.
In accordance with another aspect of the present invention, there is provided a method for inspecting an object. This method comprises the steps of: defining and associating a first and second orthogonal axes with an object to be inspected; defining a voxel in that object; passing a x-ray beam through that voxel along the first axis, and measuring incident radiation attenuation through the voxel along the first axis. While maintaining the x-ray beam aligned along the first axis, passing the x-ray beam through the object alongside the voxel and measuring scattered radiation attenuation through the voxel along the second axis. A final step consists of relating the measured incident radiation attenuation and the measured scattered radiation attenuation to a material property of the voxel.
This method is particularly advantageous because the measured incident and scattered radiation attenuations and the related material property are representative of an entirety of the voxel. The scanning of an object using this method can be done broadly, with few measurements and large voxels for example, and still provide reliable information as to the content of each voxel.
In accordance with a further aspect of the present invention, there is provided a method for inspecting an object, and which comprises the steps of: defining and associating a first, second and third orthogonal axes with an object to be inspected; defining a voxel in that object, and passing a x-ray beam through the voxel along the first axis. The method also comprises the steps of: measuring incident radiation attenuation through the voxel along the first axis; measuring scattered radiation through the voxel along the second axis, and measuring scattered radiation through the voxel along the third axis. Further steps are: using the measured scattered radiation along the second and third axes, verifying the incident radiation attenuation along the first axis, and extracting volume imaging characteristics of that voxel from the measured radiation attenuations along the three axes.
This second method provides volume imaging characteristics along three orthogonal axes without rotating the object. These characteristics are particularly advantageous for generating 3D images of structural details inside each voxel.
Other advantages and novel features of the present invention will become apparent from the following detailed description.